An investigation of the effect of porosity and diluents on micropyretic synthesis
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I.
INTRODUCTION
COMBUSTION synthesis (CS), also called micropyretic synthesis, is one o f the new processing techniques for the manufacture o f ceramics, composites, and intermetallics. The synthesis process involves the passage o f a combustion front along the specimen either smoothly with constant velocity o r in an oscillating manner. The nature o f such propagation depends on various parameters which depend on porosity, sample size and shape, reactant particles, chemical composition o f the sample, initial temperature o f the compact, composition o f the environment, and other external constraints. The critical variables w h i c h effect the propagation are combustion temperature and velocity. Further, these variables are effected by activation energy E, preexponential constant K0, heat o f combustion Q, conductivity K * , and density p o f reactants and products. The numerical study o f steady-state propagation o f an exothermic reaction front in gasless compositions was previously investigated. Exl There have been numerous observations o f oscillatory, spinning, and other timedependent modes o f combustion. [2'3] High values o f activation energy or weak exothermic reactions [21 have resulted in changing the combustion mode from steady state to oscillatory. F o r Ni-A1t31 intermetallic compound,
M.G. LAKSHMIKANTHA, Graduate Research Assistant, and J.A. SEKHAR, Associate Professor and Director, are with the Department of Materials Science and Engineering, International Center for Micropyretics, University of Cincinnati, Cincinnati, OH 45221-0012. Manuscript submitted May 1 8 , 1992. METALLURGICAL TRANSACTIONS A
it has been found that the combustion rate reaches a maximum with an equiatomic composition during steadystate combustion conditions. The combustion temperature also follows a similar trend. [4,51 Holt and M u n i rt61 have found that the addition o f l0 pct diluent TiC to the stoichiometric mixture o f T i and C causes a decrease in the combustion temperature from 2720 to 2518 K. Merzhanov [7] has reported that in the silicon-nitrogen system, the product consists o f alternating bands o f silicon nitride and elemental silicon. These bands can be easily divided into discs with the diameters e q u a l to that o f the sample.J8] It has been found that the frequency o f the oscillations is affected by the degree o f dilution o f the initial mixture by inert additions and the density and diameter o f the sample, t9'1°1 There also have been numerous numerical studies on the conditions o f thermal stability o f steady-state combustion regimes, and for many systems, these conditions have been established,tH-2°[ The stability limit for the propagation o f a pulsating exothermic reaction front in the condensed phase was also established by Shkadinskii e t al.t21j They have shown that the boundary between stable and oscillatory combustion can be described by a function a = R T c / E [ 9 . 1 ( C p T c / Q ) - 2.5] (see Nomenclature for the list o f symbols). Oscillating combustion takes p l a c e for a < 1, an
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